Simulcasting MIMO communication system

ABSTRACT

A wireless multiple-input multiple output (MIMO) communication system includes signaling simulcasting. Base stations include a plurality of transmit antennas and terminals include a plurality of receive antennas to form MIMO channels. In one embodiment, a simulcasting MIMO wireless communication system includes orthogonal frequency division multiplexing (OFDM). This arrangement achieves the spectral efficiency advantages of OFDM and simulcasting.

This application is a continuation of U.S. patent application Ser. No.13/372,313, filed Feb. 13, 2012, now U.S. Pat. No. 8,705,452 and is acontinuation of U.S. patent application Ser. No. 09/935,069, filed Aug.22, 2001, now U.S. Pat. No. 8,116,260, all of which are incorporatedherein by reference in their entirety.

CROSS REFERENCE TO RELATED APPLICATION AND STATEMENT REGARDING FEDERALLYSPONSORED RESEARCH

Not Applicable

FIELD OF THE INVENTION

The present invention relates generally to communication systems and,more particularly, to wireless communication systems.

BACKGROUND OF THE INVENTION

A wide variety of wireless communication techniques can be used totransmit and receive data between a transceiver, e.g., a base station,and a terminal, e.g., a mobile phone or station. Exemplary network typesinclude time division multiplexing (TDM), frequency division (FDM), andcode division. Each of these systems has concomitant advantages anddisadvantages. For example, single carrier systems, such as TDM and FDM,suffer problems from signal delay spread, which can degrade systemperformance and impact the overall efficiency.

Simulcasting techniques for wireless communication are well known in theart. In general, a plurality of transmitting stations eachsimultaneously transmits a given signal from the same frequency (FDM)and/or time (TDM) slots. Users within areas covered by the simulcastingtransmitters receive the simulcast signals. When near cell boundaries, auser will receive a simulcast signal from each base station serving aneighboring cell. Simulcasting enhances coverage and spectrum efficiencyas compared to systems that broadcast a given signal on differentchannels for each user requesting the signal when the same signal isrequested by multiple users.

However, simulcasting systems suffer some of the same disadvantages asnon-simulcasting technologies. For example, single carrier simulcastingsystems typically suffer problems from signal delay spread andco-channel interference and limited frequency re-use for non-simulcastsignals. Furthermore, delay spread can be even longer in simulcastsystems since the signal is transmitted by many base stations which maybe located at a range of distances from a mobile receiver.

Multiple-input multiple-output (MIMO) is another technique thatincreases spectral efficiency. In MIMO systems, multiple transmitantennas transmit different signals, all of which are separated anddetected by multiple receive antennas. In general, with M receiveantennas, up to M signals, either MIMO or co-channel interferingsignals, or a combination thereof, can be separated and detected and/orsuppressed at the receiver. Thus, when co-channel interference is notpresent, the use of N transmit and M receive antennas results in anincrease in link capacity of the minimum of N and M, i.e., if N lessthan or equal to M, an N-fold increase in capacity, theoreticallywithout any increase in total transmit power. However, N-fold MIMOincreases the number of co-channel interferers N-fold, requiring anN-fold increase in the number of receive antennas to suppress theco-channel interference. Alternatively, for a given number of receiveantennas, the degree of MIMO permitted in a system is reduced withaggressive frequency re-use, if MIMO is permitted at all.

It would, therefore, be desirable to provide a wireless simulcastingcommunication system that overcomes the aforesaid and otherdisadvantages.

SUMMARY OF THE INVENTION

The present invention provides a wireless multiple-input multiple-output(MIMO) communication system having simulcasting capability. Thisarrangement provides a spectrally efficient system that combines theadvantages of MIMO and simulcasting techniques since simulcasting haslimited co-channel interference (CCI), MIMO can be used to its fullestcapability. While the invention is primarily shown and described inconjunction with a wireless cellular system, it is understood that theinvention is applicable to wireless systems in general, in whichspectral efficiency is desired.

In one aspect of the invention, a wireless communication system includesa plurality of base stations, each having a plurality of transmitantennas, and a plurality of mobile stations, each having a plurality ofreceive antennas. In one embodiment, each of the plurality of basestations serves a respective cell or sector. The base stations cansimulcast one or more signals to the mobile stations located throughoutthe wireless system.

In a further aspect of the invention, a wireless orthogonal frequencydivision multiplexing (OFDM) communication system includes a pluralityof simulcasting MIMO base stations for communicating with a plurality ofmobile stations. With this arrangement, the system receives theadvantages of OFDM systems (e.g., mitigation of signal delay spread) theadvantages of simulcasting systems (e.g., relatively high spectralefficiency without co-channel interference) and the full use of MIMObecause of the lack of co-channel interference.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be more fully understood from the following detaileddescription taken in conjunction with the accompanying drawings, inwhich:

FIG. 1 is a schematic representation of a wireless MIMO communicationsystem having simulcasting capability in accordance with the presentinvention;

FIG. 2 is a schematic representation showing an exemplary embodiment ofa wireless MIMO communication system having simulcasting capability inaccordance with the present invention;

FIG. 3 is a schematic representation of a wireless MIMO OFDM systemhaving simulcasting capability in accordance with the present invention;

FIG. 4 is an exemplary OFDM MIMO station that can form a part of asimulcasting MIMO communication system in accordance with the presentinvention;

FIG. 5 is a graphical depiction of a subcarrier that can be used in theOFDM MIMO system of FIG. 4;

FIG. 6 is a graphical depiction showing the orthogonal nature of thesubcarriers of FIG. 5; and

FIG. 7 is an exemplary OFDM MIMO system that can form a part of asimulcasting MIMO communication system in accordance with the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a wireless communication system 100 providingmultiple-input multiple-output (MIMO) data communication betweensimulcasting base stations BS1-BS7, each of which covers a respectivecell or sector C1-C7, and mobile stations MS1-MSX and/or fixed-locationterminals FT1-FTY. As described in detail below, the base stations BSinclude a plurality of transmit antennas that form respective MIMOchannels with receive antennas located on the mobile stations MS andfixed terminals FT. The system achieves the advantages of simulcastingand MIMO systems. In one particular embodiment, the communication systemutilizes orthogonal frequency division multiplexing (OFDM) to minimizethe effects of signal delay spread, further enhancing the systemspectral efficiency.

Simulcasting is well known to one of ordinary skill in the art. Ingeneral, simulcasting refers to the broadcast of a given signal by aplurality of base stations BS or transmitters. Mobile stations MS and/orfixed terminals FT located within the cells covered by the base stationsBS receive the signal. Near cell boundaries, a user can receive multipleversions of simulcast signals, which can improve the system performanceby providing better coverage with a stronger signal, as well as betterperformance because of the lack of co-channel interference. Simulcastingis useful in a variety of network configurations. For example, a highdemand channel, such as real-time stock quotes, can be broadcast by eachbase station serving at least one user that desires to receive thechannel. In another embodiment, a network can simulcast a signal from aseries of low power transmitters, which can be located in variousbuildings, to provide coverage for users anywhere within the buildings.It is understood that the network can selectively simulcast based onuser channel demand or can constantly simulcast in predetermined areaswithout regard for user location.

As is also known in the art, conventional cellular networks have apredetermined re-use factor, such as seven for single carrier systems,for non-simulcast channels. The re-use factor defines the number ofcells in a pattern that minimizes co-channel interference. Each basestation utilizes a subset of channels to avoid use of the same channelswithin a predetermined distance. That is, base stations using the samechannels should be sufficiently spaced apart so as to minimizeco-channel interference. It is understood that simulcast channels do notgenerate co-channel interference with each other since the same signalis broadcast from different locations in the same frequency/time slots.In addition, since a mobile station located near a cell boundary canreceive two relatively weak versions of the same signal, overall systemperformance can be boosted, e.g., a 3 dB power improvement, coverage canbe more uniform, and handoffs are avoided.

FIG. 2 shows an exemplary MIMO system 200 that can form a part of awireless MIMO simulcasting communication system in accordance with thepresent invention. The MIMO system 200 can be provided from a variety ofwireless network types including time division multiple access (TDMA),code division multiple access (CDMA), frequency division multiple access(FDMA), and orthogonal frequency division multiplexing (OFDM). It isunderstood that other known and now unknown wireless networktechnologies can be used to provide a MIMO simulcasting system withoutdeparting from the present invention.

In general, the MIMO system 200 includes a plurality of transmitantennas TA1-TAN, each of which broadcasts a respective signal S1-SN. Adata stream, which can comprise one or more signals, is encoded by anencoding system 202, which generates the respective signals S1-N forbroadcast by the transmit antennas TA1-TAN. The transmitted signals arereceived by a plurality of receive antennas RA1-RAM associated with aterminal, such as a mobile station. It is understood that the number ofreceive antennas is not necessarily equal to the number of transmitantennas. The receive antennas RA1-RAM receive the transmitted signalsand provide the signals to a decoding system 204 for signal detectionand decoding.

Exemplary wireless MIMO systems are shown and described in Chevreuil,A., Vandendorpe, L., “MIMO MMSE-DFE: a General Framework,” StatisticalSignal and Array Processing, 1998. Proceedings., Ninth IEEE SP Workshopon, 1998, pages: 368-371, Ruly Lai-U Choi; Letaief, K. B.; Murch, R. D.,“MIMO CDMA Antenna Systems,” 2000 IEEE International Conference onCommunications, Volume: 2, 2000 Pages: 990-994 vol. 2, and Jian Yang;Roy, S., “On Joint Transmitter and Receiver Optimization forMultiple-Input-Multiple-Output (MIMO) Transmission Systems,” IEEETransactions on Communications, Volume: 42 Issue: 12, December 1994Pages: 3221-3231, all of which are incorporated herein by reference.

FIG. 3 shows an exemplary MIMO OFDM system 300 with simulcasting inaccordance with the present invention. The system includes a pluralityof MIMO OFDM base stations BS1-BSN simulcasting a signal to a MIMOterminal, such as a mobile station MS1. With this arrangement, theadvantages of OFDM systems and simulcasting systems are realized. Moreparticularly, the system 300 obtains the benefits of minimal co-channelinterference and increased signal power provided by simulcasting andlack of signal delay spread problems provided by OFDM systems so as toenhance the overall spectral efficiency of the system.

Referring briefly to FIGS. 4-6, an exemplary MIMO OFDM system 400, whichcan form a part of a simulcasting MIMO system in accordance with thepresent invention, includes subsystems for transmission and reception ofdata. A coding subsystem 402 encodes binary data from a data source. Thecoded data is interleaved by an interleaving subsystem 404 and thenmapped onto multi-amplitude multi-phase constellation symbols by amapping subsystem 406. In one particular embodiment, the multi-amplitudemulti-phase constellation symbols include quadrature phase shift keying(QPSK) symbols. Pilot signals can then be inserted by a pilot insertionsubsystem 408 to estimate the channel at the remote subscriber unitreceivers. A serial-to-parallel conversion subsystem 410 converts theserial data stream to a parallel data stream that is provided to aninverse fast Fourier transform (IFFT) subsystem 412.

The transformed data is converted to serial data stream by aparallel-to-serial converter 414. Cyclic extension and windowing can beadded by a subsystem 416 prior to digital-to-analog conversion by a DAC418 and transmission by an antenna system 420 including a plurality oftransmit antennas TA. The receive portion 422 of the OFDM systemincludes similar corresponding components for extracting the data fromthe received OFDM signal.

As shown in FIG. 5, the OFDM system utilizes an overlapping orthogonalmulticarrier modulation technique having a plurality of subcarriers 450.FIG. 6 shows the orthogonal nature of the subcarriers. Moreparticularly, each of four subcarriers 460 of one OFDM data symbol hasan integral number of cycles in the interval T. The number of cyclesbetween adjacent subcarriers differs by one.

FIG. 7 shows an illustrative MIMO-OFDM system 500 having multiple (hereshown as four) transmit antennas TA1-4 and a plurality of receiveantennas RA1-P. A data stream is split into first and second signalsthat are transmitted by respective pairs of transmit antennasTA1,TA2:TA3,TA4. Although the MIMO-OFDM system is shown having fourtransmit antennas, it is understood that any number of transmit antennascan be used. In addition, the number of receive antennas can bedifferent from the number of transmit antennas.

The MIMO-OFDM system 500 includes a first space time encoder STE1 thatreceives a first data block b₁[n,k] and a second space-time encoder STE2that receives a second data block b₂[n,k]. At time n at tone k, each ofthe two data blocks, {b_(i)[n,k]:k=0, 1, . . . } for i=1 and 2, istransformed into two signals, {t_(2i+j)[n,k]:k=0, 1, . . . , & j=1, 2}for i=1 and 2, respectively, through the first and second space-timeencoders STE1,STE2. Each of the coded signals forms an OFDM block. Thetransmit antennas TA1-4 transmit the OFDM signals after respectiveinverse fast Fourier transform IFFT1-4 modulation by respective signalstm_(i)[n,k] for i=1, . . . , 4.

The signals sent by the transmit antennas TA1-4 are received by thereceive antennas RA1-RAP. The received signals r₁[n,k], r₂[n,k], . . . ,r_(P)[n,k] are transformed by respective fast Fourier transform (FFT)subsystems FFT1-FFTP to generate signals that are provided to aspace-time processor STP, which provides detected signal information torespective first and second space-time decoders STD1,STD2. A channelparameter estimator CPE receives the transformed signals from whichchannel parameter information is determined and then provided to thespace-time processor STP for use in decoding the signals.

To achieve transmit diversity gain and detection of the transmittedsignals, the space-time processor STP extracts the required signals fordecoding by the first and second space-time decoders STD1, STD2. Thespace-time processor and space-time decoders each require channel stateinformation. In one embodiment, the CPE utilizes conventional trainingsequences to exploit time and frequency domain correlations of thechannel parameters. Further details of the MIMO-OFDM system 500 of FIG.7 are provided in U.S. patent application Ser. No. 09/791,523, filed onFeb. 23, 2001, now issued as U.S. Pat. No. 7,068,628, which isincorporated herein by reference.

By combining simulcasting with MIMO OFDM, the wireless communicationsystem benefits from the spectral efficiency and minimal co-channelinterference of simulcasting to enhance MIMO and the mitigation ofsignal delay spread of the multicarrier OFDM signals.

In an alternative embodiment, a simulcasting MIMO system includes TDMand FDM. The system can transmit both simulcast and non-simulcastsignals based upon whether users in the coverage area demand the samesignals. Under certain conditions, it may be desirable to reduce thenumber of MIMO channels to reduce co-channel interference ofnon-simulcast signals. More particularly, when simulcasting is used overa limited area, with other cells re-using the frequencies, then thelevel of co-channel interference into the simulcasting system can behigher (particularly near the simulcasting area boundary) and a reduceddegree of MIMO may be used.

One skilled in the art will appreciate further features and advantagesof the invention based on the above-described embodiments. Accordingly,the invention is not to be limited by what has been particularly shownand described, except as indicated by the appended claims. Allpublications and references cited herein are expressly incorporatedherein by reference in their entirety.

What is claimed:
 1. A method for wireless communication, the methodcomprising: transmitting a first signal from a first transmit antennaassociated with a first multiple-input multiple-output transmittingstation, wherein the transmitting the first signal is performed byutilizing orthogonal frequency division multiplexing; and transmitting asecond signal from a second transmit antenna associated with the firstmultiple-input multiple-output transmitting station, wherein thetransmitting the second signal is performed by utilizing the orthogonalfrequency division multiplexing, wherein the first signal and the secondsignal are transmitted while the first signal is transmitted from afirst transmit antenna associated with a second multiple-inputmultiple-output transmitting station, and while the second signal istransmitted from a second transmit antenna associated with the secondmultiple-input multiple-output transmitting station, where the firstsignal transmitted from both the first multiple-input multiple-outputtransmitting station and the second multiple-input multiple-outputtransmitting station is simulcast in a first same frequency, where thesecond signal transmitted from both the first multiple-inputmultiple-output transmitting station and the second multiple-inputmultiple-output transmitting station is simulcast in a second samefrequency.
 2. The method of claim 1, further comprising: receiving thefirst signal and the second signal on a first antenna associated with afirst terminal; and receiving the first signal and the second signal ona second antenna associated with the first terminal.
 3. The method ofclaim 1, wherein the first multiple-input multiple-output transmittingstation and the second multiple-input multiple-output transmittingstation are part of a wireless cellular system.
 4. The method of claim1, wherein the first signal is simulcasted for a high demand channel. 5.The method of claim 4, wherein the high demand channel is for providingstock quotes.
 6. The method of claim 1, wherein the first multiple-inputmultiple-output transmitting station and the second multiple-inputmultiple-output transmitting station are deployed in a building.
 7. Themethod of claim 1, wherein the first multiple-input multiple-outputtransmitting station is deployed in a first building and the secondmultiple-input multiple-output transmitting station is deployed in asecond building.
 8. The method of claim 1, wherein the orthogonalfrequency division multiplexing utilizes overlapping orthogonalmulticarrier modulation.
 9. The method of claim 8, wherein theoverlapping orthogonal multicarrier modulation comprises a plurality ofsubcarriers.
 10. The method of claim 1, wherein the second signal issimulcasted for a high demand channel.
 11. A method for wirelesscommunication, the method comprising: simulcasting a first signal and asecond signal from a plurality of multiple-input multiple-output basestations; and non-simulcasting at least one other signal from theplurality of multiple-input multiple-output base stations, wherein thesimulcasting the first signal comprises transmitting the first signal ina first same frequency, wherein the simulcasting the second signalcomprises transmitting the second signal in a second same frequency,wherein orthogonal frequency division multiplexing is utilized totransmit the first signal and the second signal that are simulcast andthe at least one other signal that is non-simulcast.
 12. The method ofclaim 11, wherein the plurality of multiple-input multiple-output basestations is part of a wireless cellular system.
 13. The method of claim11, wherein the simulcasting the first signal comprises simulcasting thefirst signal for a high demand channel.
 14. The method of claim 13,wherein the high demand channel is for providing stock quotes.
 15. Themethod of claim 11, wherein the plurality of multiple-inputmultiple-output base stations is deployed in a building.
 16. The methodof claim 11, wherein the plurality of multiple-input multiple-outputbase stations is deployed in a plurality of buildings.
 17. A wirelesscommunication system, comprising: a terminal comprising a plurality ofreceive antennas for receiving a plurality of transmitted signals,wherein each of the plurality of transmitted signals is transmitted viaa respective one of a plurality of transmit antennas included in arespective one of a plurality of multiple-input multiple-output basestations, wherein the plurality of multiple-input multiple-output basestations simulcasts a first signal from the plurality of transmitantennas in a first same frequency, wherein the plurality ofmultiple-input multiple-output base stations simulcasts a second signalfrom the plurality of transmit antennas in a second same frequency,wherein the plurality of transmitted signals is transmitted utilizingorthogonal frequency division multiplexing.
 18. The wirelesscommunication system of claim 17, wherein the terminal is a mobilephone.
 19. The wireless communication system of claim 17, wherein thewireless communication system is a wireless telephone network.
 20. Thewireless communication system of claim 17, wherein the terminalcomprises a mobile station.